The present invention relates to a method of bonding copper and a resin together with consistently high bonding strength and acid resistance, and particularly to a method of mutually bonding copper and a resin, which is suitable for production of multilayer printed wiring boards by lamination of a copper wiring layer and a resin layer.
Since no sufficient bonding strength between a metal and a resin can be secured by directly bonding the resin onto an even surface of the metal, a known method has heretofore been employed in which an oxide layer is formed on the surface of a metal to improve the bonding strength, as described in "Plating and Surface Finishing" vol. 69, No. 6, pp. 96-99 (June, 1982). However, the method has involved a problem that many metallic oxides are, in general, readily hydrolyzed upon contact with an aqueous acidic solution to be dissolved therein as metallic ions.
A variety of methods have heretofore been investigated on the surface treatment of copper for bonding of copper and a resin. However, a sufficient bonding strength is hardly secured by bonding a resin onto the surface of copper as it is. Accordingly, investigations have been made on a copper surface treatment method in which an oxide layer consisting of cuprous oxide, cupric oxide, or the like is formed on the surface of copper according to any one of various procedures to improve the bonding strength. For instance, a method of forming an oxide layer has been known in which a copper surface to be subjected to a bonding operation is treated with an aqueous alkaline solution containing potassium persulfate or sodium chlorite. Such a method of forming a copper oxide layer is very effective in improving the bonding strength. However, a copper oxide is generally readily hydrolyzed and dissolved upon contact with an acid. Therefore, contact of the oxide with the acid after the treatment must be avoided till a bonding operation. Furthermore, it has been pointed out as regards the method that, when an acid treatment is conducted after the bonding operation, there arises a phenomenon unfavorable to bonding that the oxide layer in the inner wall portions thereof around holes running through the bonding interface and in the proximity of an exposed bonding interface in a cut end portion is dissolved and soaked with the acid to result in loss of the oxide layer in the bonding interface. This phenomenon has presented a serious problem particularly in mutual bonding of a copper wiring layer and a prepreg resin in a lamination process, in production of multilayer printed wiring boards, comprising a step of perforation to form through-holes running through the bonding interface after the bonding operation and a step of various acid treatments to effect through-hole plating.
Japanese Patent Laid-Open No. 153797/1981 includes a statement that the problematic acid resistance was improved by a method comprising forming cupric oxide on the surface of copper and reducing the cupric oxide to cuprous oxide. Generally speaking, cuprous oxide is elss soluble in an acid than cupric oxide. However, even cuprous oxide is not insoluble in an acid. Thus the improvement of the acid resistance is quite insufficient.